(1) Field of the Invention
The present invention relates to a switching system of optical transmission lines in an optical communication system for protecting a communication between terminal stations. The switching system according to the present invention is applied to an optical communication system comprising at least one working transmission line and a protection transmission line.
(2) Description of the Related Art
FIG. 1 is a diagram showing a conventional construction of an optical communication system comprising two terminal stations and a plurality of working optical transmission lines and a protection (stand-by) optical transmission line provided between the terminal stations. In FIG. 1, reference numeral 910, 911, . . . 91N each denote a multiplexer/demultiplexer in a terminal station A, 920 denotes a protection switch in the terminal station A, 910', 911', . . . 91N' each denote a multiplexer/demultiplexer in a terminal station B, 920' denotes a protection switch in the terminal station B, L.sub.0 denotes a protection transmission line, L.sub.1, L.sub.2, . . . L.sub.N each denote a working transmission line, TS1, TS2, . . . TSN and TS1', TS2', . . . TSN' each denote a group of transmission lines of tributary signals of low transmission rates, and LPS and LPS' each denote a group of transmission lines for transmitting tributary signals of low priorities, where each of the transmission lines are assumed to be a bidirectional transmission line. Each of the above multiplexer/demultiplexers 910, 911, . . . 91N, and 910' , 911', . . . 91N' corresponding to each optical transmission line, is contained in one piece of terminal equipment.
In the construction of FIG. 1, the protection switches 920 in the terminal station A are provided on the sides of groups TS1, TS2, . . . TSN of transmission lines of tributary signals of low transmission rates, with regard to the multiplexer/demultiplexers 910, 911, . . . 91N, and 910', 911', . . . 91N', and connects the groups TS1, TS2, . . . TSN of transmission lines transmitting tributary signals of low transmission rates to the corresponding multiplexer/demultiplexers 911, . . . 91N, respectively in a normal condition. Thus, the respective groups TS1, TS2, . . . TSN of tributary signals which are to be transmitted from the terminal station A to the terminal station B, are multiplexed in the corresponding multiplexer/demultiplexers 911, . . . 91N to electric signals of high transmission rates. Then, the electric signals of high transmission rates which are generated in the respective multiplexer/demultiplexers 911, . . . 91N, are converted to optical signals in E/O (electric signal to optical signal) conversion circuits (not shown) provided on the sides of the optical transmission lines L.sub.1, L.sub.2, . . . L.sub.N, to be transmitted through the optical transmission lines to the terminal stations B. Parallel to the above operation of transmission, optical signals transmitted through the optical transmission lines L.sub.1, L.sub.2, . . . L.sub.N, are respectively converted to electric signals in O/E (optical signal to electric signal) conversion circuits (not shown) provided on the sides of the optical transmission lines L.sub.1, L.sub.2, . . . L.sub.N of the corresponding multiplexer/demultiplexers 911, . . . 91N, and are then respectively/demultiplexed in the corresponding multiplexer/demultiplexers 911, . . . 91N to tributary signals to be transmitted through the groups TS1, TS2, . . . TSN of transmission lines transmitting tributary signals of low transmission rates. Similar signal transmission and reception operations are performed in the terminal station B.
When a trouble occurs in one of the optical transmission lines L.sub.1, L.sub.2, . . . L.sub.N, the transmission lines transmitting tributary signals of low transmission rates corresponding to the above transmission line of the trouble, are connected to the multiplexer/demultiplexer 910 so that the tributary signals to be transmitted from the terminal station A to the terminal station B are multiplexed in the multiplexer/demultiplexer 910 to an electric signal of a high transmission rate. Although the tributary signals of the low priorities from the above group LPS of transmission lines, are multiplexed and transmitted through the protection transmission line L.sub.0 , the tributary signals of low priorities are discarded in this case. The electric signal of a high transmission rate is converted to optical signals in an E/O conversion circuit (not shown) which is provided on the side of the optical transmission lines L.sub.0, to be transmitted through the optical transmission lines to the terminal stations B. Parallel to the transmission, optical signals transmitted through the optical transmission lines L.sub.0, are converted to electric signals in an O/E conversion circuit (not shown) provided on the side of the optical transmission lines L.sub.0 of the multiplexer/demultiplexer 910. The electric signal is demultiplexed in the multiplexer/demultiplexer 910 to tributary signals of low transmission rate which are to be transmitted through the above transmission lines transmitting tributary signals of low transmission rates, corresponding to the above transmission line of the trouble. Similar signal transmission and reception operations are performed in the terminal station B.
Namely, according to the construction of FIG. 1, the switching operation from the working transmission line to the protection transmission line is carried out on the side of the tributary signals (of the low transmission rates). Therefore, the switching operations must be carried out for a large number of transmission lines transmitting tributary signals of low transmission rates. Since, in practice, the respective pieces of terminal equipment corresponding to the plurality of optical transmission lines are provided in different locations in a rack or shelf of an apparatus in a terminal station, a large number of cables must be provided for connecting between these pieces of terminal equipment for the above switching operation, and therefore, the hardware construction becomes large and very complicated, as shown in FIG. 1, thereby increasing a cost, and decreasing reliability of the system.
As an attempt to solve the above problem, a construction as shown in FIG. 2 is provided. In FIG. 2, reference numeral 930, 931, . . . 93N each denote a multiplexer/demultiplexer in a terminal station A, 940, 941, . . . 94N, and 960, 961, . . . 96N each denote a parallel to serial and serial to parallel conversion circuit in a terminal station A, 950 denotes a protection switch in the terminal station A, 970, 971, . . . 97N each denote an E/O and O/E conversion circuit in a terminal station A, 930', 931', . . . 93N' each denote a multiplexer/demultiplexer in a terminal station B, 940', 941', . . . 94N', and 960', 961', . . . 96N' each denote a parallel to serial and serial to parallel conversion circuit, 950' denotes a protection switch in the terminal station B, 970', 971', . . . 97N' each denote an E/O and O/E conversion circuit in a terminal station B, L.sub.0 denotes a protection transmission line, and L.sub.1, L.sub.2, . . . L.sub.N each denote a working transmission line, where each of the transmission lines are assumed to be a bidirectional transmission line. One of the above multiplexer/demultiplexers 930, 931, . . . 93N, and 930', 931', . . . 93N', one of the parallel to serial and serial to parallel conversion circuits 940, 941, . . . 94N, and 940', 941', . . . 94N', and one of the E/O and O/E conversion circuit 970', 971', . . . 97N', corresponding to each optical transmission line, are contained in one piece of terminal equipment indicated by a rectangular drawn by dashed lines.
In the construction of FIG. 2, the protection switches 950 and 950' are each provided on the side of electric signals of the high transmission rates, with regard to the multiplexer/demultiplexers 930, 931, . . . 93N, or 930', 931', . . . 93N'. However, since the electric signals may have to be propagated for a relatively long distance from a position of a corresponding working transmission line to the protection transmission line when the electric signal is transmitted through the protection transmission line as shown in FIG. 1, it is difficult to carry out the switching operation at the stage of the electric signals of the high transmission rates which are respectively equal to the transmission rates of the corresponding optical transmission lines L.sub.1, L.sub.2, . . . L.sub.N. Therefore, the parallel to serial and serial to parallel conversion circuits 940, 941, . . . 94N, 960, 961, . . . 96N, 940', 941', . . . 94N', and 960', 961', . . . 96N' are provided on both sides of the respective protection switches 950 and 950' so that the above electric signals of the high transmission rates are respectively divided into a plurality of parallel electric signals by the parallel to serial and serial to parallel conversion circuits 940, 941, . . . 94N, and 940', 941', . . . 94N', and the switching to the protection transmission line L.sub.0 is carried out in the stage of the divided parallel electric signals.
Thus, when tributary signals are transmitted from the terminal stations A and B, the tributary signals transmitted from the above groups TS1, TS2, . . . TSN and TS1', TS2', . . . TSN' of transmission lines, are first multiplexed in the corresponding multiplexer/demultiplexers 930, 931, . . . 93N, or 930', 931', . . . 93N', and are then divided into a plurality of parallel electric signals by the parallel to serial and serial to parallel conversion circuits 940, 941, . . . 94N, and 940', 941', . . . 94N'. The protection switches 950 and 950' each connect the transmission lines transmitting tributary signals of low transmission rates corresponding to the above trouble transmission line, to the parallel to serial and serial to parallel conversion circuits 960 and 960', and connect the other transmission lines transmitting tributary signals of low transmission rates corresponding to the other transmission lines in which no trouble occurs, to the corresponding parallel to serial and serial to parallel conversion circuits 961, . . . 96N, and 961', . . . 96N'. In the parallel to serial and serial to parallel conversion circuits 960, 961, . . . 96N, and 960', 961', . . . 96N', the outputs of the protection switches 950 and 950' are converted into the above-mentioned electric signals of high transmission rates, and the electric signals are converted to optical signals in the corresponding E/O and O/E conversion circuits 970', 971', . . . 97N', 970', 971', . . . 97N' to be transmitted through the respective optical transmission lines L.sub.0, L.sub.1, L.sub.2, . . . L.sub.N.
Parallel to the above, when optical signals are transmitted from the optical transmission lines L.sub.0, L.sub.1, L.sub.2, . . . L.sub.N to terminal stations A and B, the optical signals are converted to electric signals of the high transmission rates in the corresponding E/O and O/E conversion circuits 970', 971', . . . 97N', 970', 971', . . . 97N', and then the electric signals are divided into a plurality of parallel electric signals in the corresponding parallel to serial and serial to parallel conversion circuits 960, 961, . . . 96N, and 960', 961', . . . 96N'. The divided electric signals are supplied to the protection switches 950 and 950'. The protection switches 950 and 950' supplies the divided signals which are output from the parallel to serial and serial to parallel conversion circuits 960, are supplied to one 94i (i=1 to N) of the parallel to serial and serial to parallel conversion circuits corresponding to the above-mentioned transmission lines transmitting tributary signals of low transmission rates corresponding to the trouble transmission line, and supplies the divided signals output from the parallel to serial and serial to parallel conversion circuits 961, . . . 96N, and 961', . . . 96N' are supplied to the other parallel to serial and serial to parallel conversion circuits 94j (j=1 to N, and j.noteq.i). The outputs of the protection switches 950 and 950' are converted into the above-mentioned electric signals of high transmission rates in the parallel to serial and serial to parallel conversion circuits 940, 941, . . . 94N, and 940', 941', . . . 94N', and the electric signals are demultiplexed to tributary signals of low transmission rates to transmit the tributary signals through the transmission lines of the low transmission rates.
However, even in the construction of FIG. 2, the switching operation from the working transmission line to the protection transmission line is carried out at the stage of the above-mentioned divided signals. Therefore, the switching operations still must be carried out for a large number of divided parallel transmission lines. Since, in practice, the respective pieces of terminal equipment corresponding to the plurality of optical transmission lines are provided in different locations in a rack or shelf of an apparatus in a terminal station, a large number of cables must be provided for connecting between these pieces of terminal equipment for the above switching operation, and therefore, the hardware construction becomes large and very complicated, as shown in FIG. 2, thereby increasing cost, and decreasing reliability of the system.